Table of contents

A nonvisual, optical method is presented for separate measurement of the width of oppositely magnetised domains in stripe domain structures in transparent uni-axial magnetic layers. Comparison is made with visually measured values and it is found that the nonvisual method yields better values for the average domain width.

The Hall coefficient R_H of specimens of cadmium mercury telluride, suitably doped with acceptors, appear anomalously n-type at low temperatures (circa 50K and below). This has been shown to correlate with the extent of surface damage and oxidation. A two-layer model has been used to predict the double sign change and anomalous low temperature R_H and is in good agreement with the experimental results.

Magnetic domain structures in thin foils may be revealed in a number of ways in the transmission electron microscope. The most commonly used modes (Fresnel, Foucault, holographic, differential phase contrast and low angle diffraction) are described and intercompared. The Fresnel and Foucault modes are simple to implement but provide a mainly qualitative description of the spatial variation of induction throughout the foil. Quantitative information may be obtained from the other two imaging modes but they require more sophisticated equipment. Electron diffraction is particularly useful for studying periodic magnetic structures. Examples of the use of these, and further specialised techniques, are presented by reference to some applications of current interest within magnetism.

Theoretical and experimental investigations of the electromechanical electret slit-effect transducer are presented. After deriving the basic equations of the bidirectional (or reciprocal) transducer from known relations for electret fields, the force and the electric response are calculated for a very general case. From these fundamental equations, the responses of slit-effect devices with symmetric bipolar electrets, with monopolar electrets, and with one-sided metallised electrets are deduced. Experimental results confirm the theoretically predicted linearity and yield an electromechanical reciprocity constant of the order of 1 mu NV^-1 for a single-element slit-effect transducer. Furthermore, force and dynamic voltage-response measurements of a 20-element transducer are reported. Finally, technological problems and possible applications of slit-effect devices are discussed.

The use of ring-gyroscopes to detect certain types of general relativistic gravitational field effects is considered using a generalised form of Lenz's law.

The calibration of a laser interferometry system for acoustic emission measurements proposed by Palmer et al. (1977) is shown to have overestimated the displacement. The correct analysis has shown that their overestimate is by a factor of 0.5819.

A new model is presented which describes the adhesion of a particle to a surface following an elastic-plastic impact. The experimental testing of the model is described and it is shown that use of the model leads to values of the adhesive surface energy between the particle and the surface which are in good agreement with values obtained by other means. A method for determining the adhesive energy from the measurement of impacts involving only elastic deformations is also presented.

The breakdown of helium, produced by single-mode laser beam with a triangular time-variation, has been calculated on the basis of the cascade mechanism by following the evolution of the electron energy distribution. A simple model of the helium atom has been used-a ground state, one excited and one ionised state, and the collision cross-sections for ionisation, excitation, and ionisation of the excited state have been taken from the literature. With parameters corresponding to a focal spot of diameter 14 mu m, in helium of pressure between one and 12 atm, and no loss processes, the computed breakdown threshold intensities were found to be within a factor of two of those determined experimentally. On making allowance for diffusion losses, the agreement improved to the order of 20%.

The diffusion tensor in an isolated electron or light ion swarm subject only to elastic collisions with molecules has been formulated from the multiterm expansion of the Boltzmann equation in the hydrodynamic description. Derived diffusion coefficients for the transverse and longitudinal directions include the additional terms representative of the curvature effect under the action of an electric field besides the usual two-term expressions. Discussion is, particularly, focused on the relation between the transverse diffusion coefficient D_T and its conventional expression D_V. Numerical calculations have been made for the electron swarm in a model gas having a momentum transfer cross-section, Q_m( epsilon )=Q₀ epsilon ^beta ( beta =0, ¹/₂, 1), with isotropic scattering. It will be recognised that an appreciable degree of discrepancy occurs between D_T and D_V with the increase of the power beta , i.e. of the cross-sectional dependence on energy.

A systematic electron microscopy analysis is performed of the defects on metallic surfaces, either pre-existing or introduced by processing or as a result of powerful laser irradiation. Metallic and nonmetallic defects were revealed with a vaporisation threshold lower than that of an ideal plane metallic surface. This lower threshold might lead to a lowering of the breakdown threshold of a gas in the neighbourhood of the metallic target.

The measurement of electron densities using interferometry and refraction measurements in plasmas with cross-sections where the electron density contours are concentric ellipses is examined. Transforms are found for both interferometrically deduced optical path-length differences and refraction angle data obtained from elliptical cross-section plasmas, which in the limit of small amounts of refraction adjust the data to values which appear as if they were obtained from equivalent circular cross-section plasmas. The transformed data can be inverted to give electron densities using standard techniques developed for circular cross-section plasmas (e.g. Abel inversion). To check the accuracy of the transforms for moderate to large amounts of refraction (>or=0.1 rad), refraction of light in elliptical cross-section plasmas is examined using numerical ray tracing.

It has been shown that the quasi-equilibrium radius ratio kappa _p of a fast compressional pinch is independent of the absolute magnitude of the driving current; it depends only on the current shape and on the length variation of the pinch during pinching. In this paper it is shown that a current step, switched on to the discharge current of a pinch as it approaches its quasi-equilibrium radius ratio kappa _p as defined by a lossless energy balance theory, shifts the value of kappa _p to a significantly lower value, thus enhancing pinch compression. It is proposed that this current-stepping technique may be used as a prelude to radiation cooling to eventually achieve the very small radius ratios required for a pinch fusion reactor.

The motion of vacuum arc cathode spots under the influence of self-generated azimuthal and externally applied axial magnetic fields has been investigated for several metals at currents up to about 10 kA. The spot motion was recorded by means of a high-speed framing camera, and the self-generated magnetic field was measured with an inductive magnetic probe. The axial magnetic field was applied by means of two coils round the vacuum chamber in an approximate Helmholtz arrangement. The data have been used to determine the relationships between the retrograde (anti-Amperian) spot velocity and the self and axial magnetic flux densities. At low self-generated fields, the velocity is directly proportional to the flux density but at high fields velocity saturation is observed in some cases. An axial magnetic field decreased the retrograde velocities on all metals investigated. The spontaneous spot formation observed with high rates of current rise is analysed; the velocity saturation phenomenon is explained semiquantitatively and the influence of the axial magnetic field on the cathode spot velocity discussed.

Arcing voltage characteristics of metal vapour arcs in vacuum for seven metals (copper, aluminium, tin, magnesium, zinc, cadmium and bismuth) of high purity and low gas content have been investigated. These metals cover a fairly wide range of thermophysical parameters. A triggered vacuum switch with circular butt contacts of 75 mm diameter and 15 mm separation was used and the discharge current was varied up to about 10 kA. The low-current arcing voltage and average current per cathode spot are reported for these metals. The empirical relationships for arcing voltages are derived for the linear range of the voltage-current characteristics. Voltage-current characteristics in axial magnetic fields up to 0.2 T are presented and the observed similarities and differences in the behaviour of the metals investigated are explained. Finally, the axial magnetic field necessary for giving a minimum arcing voltage is discussed.

The effect of the interelectrode gap on some characteristics of the glow discharge in electronegative gases (oxygen and air) at low pressures (0.1-1 Torr) has been studied in a specially constructed discharge tube with parallel plate electrode geometry. In this tube the interelectrode gap could be varied without altering the discharge pressure or the planarity of the electrodes. The interelectrode gap has been found to have a profound effect, both on the magnitude of the discharge current and its stability. The current decreased to zero, at a constant pressure and applied voltage, as the anode was gradually moved towards the edge of the cathode dark space or drawn away from the cathode beyond the Faraday dark space. This variation of the current has been discussed. Moreover, when the interelectrode gap was such that the discharge current was less than about 70% of the maximum discharge current, highly regular oscillations of frequency in the range 1-6 Hz developed. This behaviour has been explained by the formation of negative ions of the gas and their subsequent decay. When the applied voltage was so low that only a Townsend discharge formed, regular instabilities of low frequency occurred at all interelectrode gap values studied.

Employing continuum theory, this paper investigates the onset of homogeneous and roll instabilities in Couette flow of a nematic liquid crystal when a radial temperature gradient is present. The arrangement considered is that in which the anisotropic axis of the liquid crystal is initially aligned parallel to the axis of the circular cylinders. Using an orthonormalisation method, the authors present accurate numerical results for the shear rate thresholds as a function of the applied thermal gradient. It is shown that a positive thermal gradient reduces the roll threshold but that a thermal gradient of either sign has no effect on the homogeneous threshold. They therefore predict a value phi _T for the radial temperature gradient at which there is a transition from a homogeneous to a roll-type instability. In addition they demonstrate the important effect rotation has through the centrifugal force.

A thermally stimulated depolarisation (TSD) technique is presented, for studying the crystallisation of supercooled water dispersed within emulsions. The technique makes use of the high resolving power and the scanning character of the TSD method and of the phenomenon of evolution with time of the dielectric properties of ice dispersions in oil. It enables the determination of the freezing temperature, when the emulsion is submitted to steady cooling, and of the fraction of frozen water as a function of time, when the emulsion is maintained at a fixed temperature. No models or formulae for the dielectric behaviour of the emulsion are needed to this end. Examples illustrating the efficiency of the technique are also given.

The transport properties of SnTe epitaxial films of different thicknesses deposited on mica substrates at a substrate temperature of 448K have been studied at 300K. Thermoelectric power, conductivity and Hall data confirm the presence of inhomogeneity in the SnTe film-substrate interface. The data obtained from these measurements have been analysed with a two-circuit model and the thickness of the interface layer is calculated to be 600 AA.

For EEPROM devices, it is typically observed that the threshold window, defined as the difference between the erase and write threshold voltage, begins to narrow gradually until around 10⁶ cycles when the window collapses. This phenomenon of window collapse can be attributed to trapping of injected electrons in the thin tunnel oxide. The authors present a calculation which demonstrates the effect of electron trapping on window collapse.

The electrical and photovoltaic properties of Al-doped and undoped polyacetylene-Electrodag +502 Schottky barrier diodes have been examined. The current-voltage and capacitance-voltage characteristics at different temperatures were investigated. The dependence of short-circuit current and open-circuit voltage on the light intensity, temperature, concentration of acceptors and oxygen was also determined. The yield of light energy transformed into electrical energy around 0.1% is obtained in the best case (V_oc approximately=650 mV, J_sc approximately=200 mu A cm^-2, FF approximately=0.3 and P_in approximately=50 mW cm^-2).

A pulsed electron beam is used to generate electron-hole pairs in SiO₂ films and the yield measured as a function of temperature and applied voltage. It is shown that over the temperature range 200 to 400K the electron yield is controlled by an initial recombination process.

It is shown that a temperature-dependent series resistance of the back contact of the semiconducting substrate in MIS capacitors can lead to transient charging of the insulator at low temperatures. Both the position and height of the corresponding deep-level transient current spectroscopy (DLTCS) peak are shown to be functions of the insulator capacitances of individual structures on a common substrate. The experimentally observed low temperature peak of DLTCS in Al-SiO₂-Si(n) diodes is found to exhibit all the qualitative features predicted for the case of a non-negligible series resistance, the resistance of the back contact being shunted by a parallel capacitance C_s. Finally, a correlation between time and frequency domain experiments is demonstrated.

A pulsed electron beam is used to investigate charge transport in dry thermal oxides grown in the presence of HCl. The results indicate that no radiation hardening was achieved in these oxides. Furthermore, incorporation of chlorine in the oxides is apparently accompanied by significant hole trapping.

The reflection of sound at the normal zone boundary in hard superconductors and in superconducting composites is investigated. The value of the reflection coefficient is found and it is shown that it reaches a maximum at low (less than 10⁵-10⁶ s^-1) frequencies. The observation of the reflected acoustic signal allows one to find the position of the normal zone in the sample, the propagating velocity of the normal zone, and to evaluate the temperature in the normal phase.

Depth and lateral-spread distributions of defects generated by 0.5-5 keV argon beam in crystalline nickel has been determined by using the computer simulation program MARLOWE. Over the whole energy range, vacancies are well separated from the interstitials and the formation of depleted zones is clearly indicated. Densities of defects and deposited energy are evaluated and the dependence of defect configurations on some lattice parameters is discussed.